Although some people believe that taking selenium can reduce a person's risk of cancer, a Cochrane Systematic Review of randomised controlled clinical trials found no protective effect against non-melanoma skin cancer or prostate cancer. In addition, there is some indication that taking selenium over a long period of time could have toxic effects. These conclusions were reached after researchers scanned the medical literature, looking for trials that studied the effects of taking selenium supplements and observational studies on selenium intake. The researchers located 49 prospective observational studies and six randomised controlled trials.